Wood preserving compositions are well known for preserving wood and other cellulose-based materials, such as paper, particleboard, textiles, rope, etc., against organisms responsible for the destruction of wood, namely fungus and insects. Many conventional wood preserving compositions comprise copper amine complexes. Copper amine complexes have been used in the past because the copper in such complexes is soluble in aqueous solutions. The copper in the copper amine complexes is typically obtained from a variety of copper bearing materials, such as copper scrap, cuprous oxide, copper carbonate, copper hydroxide, a variety of cuprous and cupric salts, and copper bearing ores. The amine in such copper amine complexes is normally obtained from an aqueous solution of ammonia and ammonium salts such as ammonium carbonate and ammonium sulfate.
A conventional method to prepare such copper amine complexes is disclosed in U.S. Pat. No. 6,340,384. In that patent, the inventor wrote, “The copper amine complex may be prepared by methods known in the art. For example, U.S. Pat. No. 4,622,248 describes forming copper amine complexes by dissolving copper oxide in ammonia in the presence of ammonium bicarbonate.” Another method to form cupric oxide and cupric oxide amine complexes is disclosed in U.S. Pat. No. 5,492,681 to Pasek et al. Pasek et al. describe a process to produce cupric oxide from copper bearing materials with aqueous ammonia and an ammonium salt in the presence of oxygen to form a cupric amine compound which, upon heating, decomposes to cupric oxide, ammonia and water.
The disadvantage of using aqueous ammonia as copper solubilizing agent lies in the strong odor of ammonia. Additionally, copper ammonia preservatives can affect the appearance of the treated wood giving surface residues and undesirable color. In recent years, many amine-containing compounds, such as ethanolamines and aliphatic polyamines, have been used to replace ammonia to formulate water-soluble copper solutions. These compounds were chosen because of their strong complexing ability with copper and the absence of undesirable odor. U.S. Pat. No. 4,622,248 discloses a method of preparing copper amine complexes by dissolving a mixture of copper carbonate [CuCO3] and copper hydroxide [Cu(OH)2] in ethanolamine and water. The complexing amine and copper combine stoichiometrically in a 4 to 1 (molar ratio) and thus the weight ratio of reagents will be different for each complexing amine. However, copper amine based preservatives, such as copper monoethanolamine (Cu-MEA), have higher copper loss due to leaching as compared to a traditional copper based preservative chromated copper arsenic (CCA). The high copper leaching in Cu-MEA is believed to be due to the strong complex which forms between the cupric ion and monoethanolamine (MEA). For example, more than 25% of copper present in the treated wood can be leached out when wood is treated with a Cu-MEA solution containing 0.5% copper. Although not desiring to be bound by any particular theory, the inventors believe that as the amine compound, such as MEA, is leached from the treated material, the strong complex between Cu-MEA results in the leaching of copper along with the leached MEA.
U.S. Pat. No. 5,186,947 also discloses a method of preparing water soluble copper complexes using polyamines as the solubilizing agents for copper. In this invention, aliphatic acids are used in the formulations to improve the fixation and reduce the leaching of copper from wood. This patent also teaches that polyacrylic acids may be added to copper-polyethylenimine (Cu-PEI) formulations. We have discovered that the addition of polyacrylic acid to Cu-PEI results in the formation of a copper precipitate. The precipitation of copper from a wood preservative formulation is undesirable because it reduces the amount of copper present in solution which is able to penetrate into the wood. Additionally, precipitated copper will remain on the surface of the wood, potentially contributing to an uneven surface appearance of the treated wood. This is undesirable in providing an aqueous wood preservative formulation useful in providing wood products that are aesthetically acceptable to the public, yet preserved from the destructive agencies of wood. Accordingly, the discovery that certain vinyl based polymers aid in decreasing the leaching of metal from treated wood while still providing clear aqueous formulations was entirely unexpected. Thus, the need to provide aqueous copper amine complexes that are suitable for use to treat wood, yet minimize the copper leaching from treated wood when exposed to water and do not result in precipitation is addressed by the incorporation of polyethylenimine and the vinyl based polymers poly(vinyl alcohol), polyacrylamide, poly(N-vinyl pyrrolidone), and poly(N-isopropyl acrylamide) into copper amine formulations.